In electrophotographic image transfer, a pattern of electrostatic charges corresponding to an image is developed on an optical photoreceptor (OPR), such as a photosensitive drum. Toner is applied to the OPR, and that toner which is retained as a result of not being repelled by electrostatic charges forms a pattern which is transferred to an intermediate transfer belt, and then to a substrate (such as, for example, paper or plastic media). In color image forming devices, there typically will be four colors of toner (black, magenta, cyan and yellow) applied to the photosensitive drum and subsequently transferred from the drum to the intermediate transfer belt (although in some systems only three colors are used, and in other systems more than four colors can be used, with an exemplary seven-color system using black, magenta, yellow, cyan, red, green, and blue). The colors are generally transferred in the order of black, magenta, cyan, and finally yellow, with the transfer of any color not occurring until the transfer of all colors preceding that color in the listed order are finished. Accordingly, the formation of a pattern on the photosensitive drum, and subsequent transfer of the pattern to the intermediate transfer belt, occurs four times (once for each of the colors black, magenta, cyan and yellow), with the overlapping patterns on the intermediate transfer belt forming an image that is to be transferred to the substrate. It is noted that single-color patterns are typically transferred from the OPR to the intermediate transfer belt before an entirety of the single-color of toner that is ultimately to be formed on the intermediate transfer belt has been transferred to the OPR. For instance, black toner will typically begin to transfer from the OPR to the intermediate transfer belt while additional black toner is still being applied to the OPR. The toner is not, however, typically transferred from the intermediate transfer belt to the substrate until an entire image (i.e., an image containing all four of the colors of magenta, cyan, yellow and black) is formed on the intermediate transfer belt. The image formed on the intermediate transfer belt can correspond to, for example, an entirety of an image formed on a single sheet of paper. The intermediate transfer belt can be, for example, long enough to contain a complete legal document image wrapped around its circumference, plus a few extra inches. The extra space on the intermediate transfer belt between the top and bottom of the image formed on the intermediate transfer belt is called the inter-document zone.
Specific steps utilized in forming an image with a prior art image transfer device are described with reference to FIGS. 1-3. Referring to FIG. 1, a prior art image forming device 10 comprises a rotating photosensitive drum 12 (with the rotation indicated by an arrow 23) and an intermediate transfer belt 14 moving past drum 12 in a direction indicated by arrow 25. Photosensitive drum 12 carries a pattern of positive charges 16 on its surface, and such positive charges retain negatively charged toner particles 18.
A support structure 20 and a primary transfer roller 22 are provided to support intermediate transfer belt 14. Primary transfer roller 22 is in electrical connection with a DC power source 24, and is utilized to provide a positive charge to intermediate transfer belt 14. Such positive charge attracts the negatively charged toner from photosensitive drum 12 onto intermediate transfer belt 14. The FIG. 1 process of transferring toner from photosensitive drum 12 to intermediate transfer belt 14 is repeated four times in a color image transfer process (one time each for the black, magenta, cyan and yellow toners). The positive bias applied to transfer roller 22 is generally increased after each toner pass to compensate for increasing layers of toner.
FIG. 2 illustrates intermediate transfer belt 14 after an entire image has been formed on intermediate transfer belt 14. Specifically, four layers of toner 18 (only some of toner 18 is labeled in FIG. 2) are shown applied over transfer belt 14, with the layers corresponding to black, magenta, cyan and yellow toners. In the view of FIG. 2, transfer belt 14 is moved in a direction indicated by arrow 30.
FIG. 2 further shows a substrate 34 being fed through feed support structures 33 of apparatus 10 and across a secondary transfer roller 36, in a direction indicated by arrow 31. Secondary transfer roller 36 is in electrical connection with a DC power source 38. Power source 38 creates a positive charge which pulls toner 18 from intermediate transfer belt 14 onto substrate 34. After the toner is transferred to substrate 34, the toner is fused to substrate 34. Subsequently, substrate 34 exits device 10.
A difficulty in the processing of FIGS. 1 and 2 can be in maintaining consistent toner density during repeated printing operations. For instance, it is found that toner density can vary due to environmental conditions, deteriorated toner, or a deteriorated photosensitive drum. A method of monitoring and maintaining toner density is described with reference to FIG. 3.
FIG. 3 illustrates the photosensitive drum 12 of apparatus 10, and further shows a pattern of toner patches 40 (only some of the toner patches 40 are labeled) which has been provided over a surface of photosensitive drum 12. Toner patches 40 preferably vary in density relative to one another, with the densities being determined by a controller 42. Controller 42 is in data communication with a density sensor 45 which comprises a light emitting diode (LED) 44, and a pair of photodiodes 46 and 48. Density sensor 45 is utilized to read densities of toner patches 40. Specifically light is emitted from LED 44 and received directly by photodiode 48, as well as reflected from toner patches 40 to be received by photodiode 46. The signals received by photodiodes 46 and 48 are compared utilizing processing circuitry within controller 42. Controller 42 can then adjust parameters associated with toner transfer to correct for errors encountered in the densities of toner patches 40.
In spite of the above-described methodologies for correcting errors in toner density, inaccuracies in toner application (such as, for example, errors in spatial alignment) can still be found in images formed by image transfer devices of the type described with reference to FIGS. 1-3. A method of detecting such errors is to print test patterns on substrate passed through apparatus 10.
However, while such test patterns can be useful for identifying errors, it is generally time-consuming to run and utilize such test patterns. Further, it is generally desirable to utilize methodologies which can be incorporated into image forming apparatuses to automatically detect and correct toner transfer errors without human intervention. The printing of test patterns on substrates passed through an image forming device is generally difficult to incorporate into such automatic detection and correction mechanisms. Accordingly, it would be desirable to develop alternative methodologies for detecting toner transfer inaccuracies and to incorporate such methodologies into processes which can automatically detect inaccuracies in toner transfer and correct such inaccuracies.